(a) Technical Field of the Invention
This invention relates generally to filters. More specifically, it relates to a reconfigurable filter design that incorporates microelectro-mechanical switches.
(b) Description of Related Art
In signal processing and communication systems, it is often desirable to remove unwanted frequencies from a signal, or isolate and allow desired frequencies to pass. Typically, a separate filter system is required for every district range of frequencies that is desired. For example, low-pass filtering would require one filter system and band-pass filtering would require a second. If it were desired to filter frequencies at a second low-pass cutoff, a second low pass filter system would be required. In millimeterwave circuits, these filters are typically laid out on printed circuit boards using microstrip lines with filter stubs attached to the microstrip lines. The filter stubs present an impedance to the millimeterwave signals passing through the microstrip line. The magnitude and type (inductive or capacitive) of the impedance is varied by altering the physical characteristics of the filter stubs. Through a combination of stubs, a wide variety of filters can be designed.
A separate microstrip filter system is required for each district range of frequencies. If a large variety of filter configurations is desired, the filter assembly would require a large amount of layout area. For millimeterwave circuits on printed circuit boards for applications such as satellite communications, layout space is at a premium.
It is therefore desirous to have a single filter that can be configured to filter many different frequency bands. In the past, YIG spheres and varactor diodes have been used as reconfigurable filters. These filters, however, suffer from a poor insertion loss--that is, there is a significant signal loss across these devices even when the filters are set in an `all-pass` configuration. In addition, varactor filters are continuously tunable filters and have filter characteristics that are dependant upon the bias of the filter. Thus, fluctuations in the filter biasing will affect the filter characteristics. In addition, varactor filters typically cannot be used above 10 GHz because of the relatively low quality factor or "Q", of the varactor filters.
Accordingly, there is a need for a reconfigurable filter that has a low insertion loss, a large operating bandwidth, and filter characteristics that are not altered by deviations in biasing.